The effective mac user blog has posted 11 ways to optimize Mac's performance.
For example it suggests use of utilities such as OnyX which can repair and optimize Mac computer system.
From the article:
Every now and then, my Macs begin to feel a little sluggish. There are many potential reasons why: I tend to run 8-10 applications all the time - and sometimes push 15 or more. This alone will bog down any Mac.
At other times, I realize that it's been weeks since I restarted the computer, and a simple restart will solve a lot of these woes.
When those don't speed things up, I've found a number of things I can do to encourage my Macs back to their youthful snappiness.